1. Field of the Invention
The present invention relates to a discharge lamp energizing circuit for energizing a discharge lamp, e.g., an HID (High Intensity Discharge) lamp such as a high-pressure sodium lamp, a metal halide lamp, a mercury lamp, or the like, and a method of energizing a discharge lamp.
2. Description of the Related Art
Generally, HID lamps offer the following advantages:
(1) Since each HID lamp emits a large flux of light, it is suitable for illuminating a large space.
(2) Since HID lamps have a better light emission efficiency than incandescent lamps and halogen lamps, the HID lamps serve an energy saver in facilities which adopt them.
(3) Since HID lamps have a long service life, their running costs and maintenance costs are low.
One known circuit for energizing a discharge lamp such as an HID lamp or the like is an energizing circuit 1 shown in FIG. 36 of the accompanying drawings, as disclosed in Japanese Laid-Open Patent Publication No. 2003-272887, for example.
As shown in FIG. 36, the energizing circuit 1 comprises a DC power supply 2, a DC-to-DC converter 3, a DC-to-AC converter 4, a starter 5, a discharge lamp 6, a controller 7, and a detector 8.
The DC-to-DC converter 3 converts a DC voltage applied from the DC power supply 2 into a desired DC voltage. The DC-to-DC converter 3 comprises a voltage-resonant flyback DC-to-DC converter.
The DC-to-AC converter 4 converts the DC voltage output from the DC-to-DC converter 3 into an AC voltage, and supplies the AC voltage through the starter 5 to the discharge lamp 6. For example, the DC-to-AC converter 4 comprises a bridge of four semiconductor switching devices and a driver for the bridge. Two pairs of the semiconductor switching devices are complementarily turned on and off to produce an AC voltage.
The starter 5 generates a high-voltage pulse signal for starting the discharge lamp 6, i.e., a starting pulse, and applies the high-voltage pulse signal to start energizing the discharge lamp 6. The high-voltage pulse signal is added to the AC voltage output from the DC-to-AC converter 4, and then applied to the discharge lamp 6.
The controller 7 is supplied with a detected signal representative of the actual voltage applied to the discharge lamp 6 and the actual current flowing to the discharge lamp 6 or a voltage which corresponds to the actual voltage and a current which corresponds to the actual current. In response to the detected signal, the controller 7 controls electric power charged into the discharge lamp 6 and also controls the DC voltage output from the DC-to-DC converter 3.
Recently, HID lamps have been finding use as light sources for automotive headlights. Therefore, there is a growing demand for small-size energizing circuits for discharge lamps.
The conventional circuit for energizing a discharge lamp as disclosed in Japanese Laid-Open Patent Publication No. 2003-272887 poses limitations on efforts to reduce its size because it requires the starter 5 in addition to the DC-to-AC converter 4.
Furthermore, since the HID lamp needs to be kept at a high temperature for its operation, it takes about 4 to 8 minutes until the brightness becomes stable after the power supply switch thereof has been turned on. When the HID lamp is turned on immediately after it has been de-energized, it takes about 5 to 15 minutes until it starts emitting light. Particularly, an HID lamp placed in an automotive electric accessory such as a headlight tends to have a long restarting time.
Accordingly, it is difficult for HID lamps to find widespread use as light sources for automotive headlights unless they are smaller in size and can be restarted in shorter time.